"I'm in my studio today. I am presenting in one-on-one appointments, with the different editors and editor-in-chiefs, my new collection before fashion week," Zac Posen says by phone, casual as could be. For most, that would be a formidable enough day, but the 36-year-old designer and Project Runway judge is about to embark on a months-long media blitz that includes a trip to the Emmys, where the fashion design series is nominated for Outstanding Reality-Competition Program; the launch of his first cookbook, Cooking With Zac: Recipes From Rustic to Refined; and the debut of House of Z, an enchanting and surprisingly emotional documentary that tracks Posen’s journey from 21-year-old prodigy to global luxury brand.

House of Z, which features commentary from the likes of Claire Danes, Naomi Campbell and Sean “Diddy” Combs (whom Campbell still calls “Puffy”), is exclusively available to rent on Vogue.com on Sept. 6.

"It's one thing wearing many hats in the creative process,” Posen explains. “When it gets to promoting so many [projects in] various different mediums, there's an art in it. And the art is preparation. But between Project Runway and [the fact that] I work on 16 collections a year, I'm used to it at this point.”

In a conversation with ET, Posen reflects on the darker times in his career, what moved him most about the documentary and Project Runway’s decision to include models of all sizes on the runway.

ET: How did this documentary come about?

Zac Posen: The documentary came about now four and a half years ago, and it started with a small, 45-minute documentary that was being created on the making of this collection in conjunction with a charity in Toronto. Sandy Chronopoulos, the director, came in, and she comes from a journalist background. I thought the story at the time was really the making of the collection, but through the process of her capturing the collection and putting the show on, she did some interviews and started to pick up on the idea that there was a bigger story to tell here. That was her investigative journalist side. [Laughs]

At that point, what was important to her if she was going to make a feature film was that she had full control. I had to sign off on having final cut of the film or any kind of editorial control so that she could make a real documentary. I gave that OK. From somebody that is very much in control of, as much as one can be, and conscious of how one's image is being portrayed, that was a very scary and risky move. Obviously, there had been trust that had been established, but it was a nerve-wracking process. Sandy started to build the story she wanted to tell through very serious and deep research and probing me on areas that, you know, I wasn't comfortable with discussing at the time. That was all for the purpose of making another level fashion film. This is not a puff piece, it's not an advertorial. It's not my film; It's Sandy's film.

As someone who isn't as tapped into the inner workings of the fashion industry as I wish I were, until I saw the trailer for House of Z, I had no idea you ever had any sort of fall from grace. I always thought of you as that "Boy Wonder,” then you were designing for actresses and doing cameos on America’s Next Top Model, and then Project Runway. I never knew about the darker times.

There definitely were darker times. I think that I, as somebody who owns my own business, we always can continue to have growth. My working and my interactions with celebrity dressing or from a larger public standpoint never had this quote unquote setback. But I think that from the internal fashion community or reviewers -- which, now looking back at all of this, that whole section within the fashion industry has evolved greatly -- there definitely was a lot of pushback. I came against an enormous amount of inner workings of politics and also just reaction to the global economy and my coming-of-age.

Looking back at the tougher times then, did you have the urge to say, Let's leave that in the past. I'm beyond it? It can't be easy to revisit and hear some of those things that were said about you.

Of course! But beyond what things were said about me, I think I have a new perspective today, because the experience I went through was very formative to get me to who and where I am today. From a professional standpoint, my company is in an incredibly strong, productive place. From a human standpoint, I made a really clear transition -- a humbling and invigorating, I would say, transition -- to a real understanding of who I am, that my life is my work, rather than the work and fashion being a facilitator or an entrée into a lifestyle. Because that's not what this is about! Really, if you want to work, you have to be incredibly dedicated.

It also forced me to have a much further, deeper understanding and responsibility of the business side of fashion. The tough love from, obviously, what people said about me, but more the experience [of] potentially having everything taken away from you, having to really look at yourself -- what your clothing, your work, represented, what you represented, how that was misinterpreted, what parts you can be responsible for, what elements are out of your control -- can break most people, or you can refocus and find a kind of resilience out of it and be invigorated and learn from it. That's how I look at that experience. Tough love is not fun to go through. Not being understood is hard. I think attention and jealousy can be a strong component that is out of one's control, by nature. When your celebratory nature of life is questioned and frowned upon and your optimism or even ambition can be frowned upon, that can be quite challenging.

There are a lot of talking heads in this film -- your family, your friends, celebrities, critics. Is there a revelation or one thing that someone said that surprised you or stuck out to you when you watched?

The most surprisingly element of the film -- and the element that I was the most protective about and not something, initially, that I was necessarily comfortable with -- was the bigger story of this movie, the surprise story, which is the family story. The process of making the film itself was a reflective, healing process, especially with my sister. I think it takes a level of real maturity to reflect on oneself and take responsibility for actions. My sister said to Sandy after an interview, something about "Remember, this is not just a story about Zac. It's also a story about the family." And, to me, that's the moment that cuts through.

For sure, watching the movie and watching Sean Combs' interview, now with a decade of reflection on it, it just made me fall in love with Sean again. And when I hear Naomi speak, it's very clear to me why we love each other and I pinch myself at how incredibly fortunate and lucky I was [to have] the passion and drive and care that these incredibly talented and driven people invested into me as a friend and mentors and business partners early in my career.

Dimitrios Kambouris/Getty Images

You mentioned the lifestyle earlier, and we do get glimpses of the glamour of it all in the documentary. What is the most fabulous experience you've ever had? You can take a minute to think, because I really want you to paint the picture.

My most fabulous experience?! [Laughs] Well, I just want to say, I think that a lot of people enter into fashion and think it will facilitate them to live like royalty. And in some cases, it can. But at the end of the day, as a fashion designer, you're here to practice your craft and serve your client. There was a moment in my late 20s where I realized [that lifestyle] just wasn't what I valued anymore. I didn't care -- I realized it was like playing Keeping Up With the Jones. No matter what was going to happen at that time, my value system wasn't about owning a private jet. Sure, flying private is an incredible experience. But that's not what I'm about!

The most fabulous experience I've ever had? Wow... It depends! In terms of a superficial side of it, I mean, I'm weird. I think, like, really luxurious plants and vegetables are really luxurious. [Laughs] I don't know. Private islands in Greece? The luxury of being able to go to Madonna's birthday party or Christmas party. Those are cultural moments that are just fabulous. Going to her private Christmas party? Amazing! My 12-year-old self, voguing in my bedroom, would never have believed that that reality could come true.

I want to ask you about Project Runway. The twist this season is that the runway is body inclusive, with models of all shapes and sizes. You've dressed many different types of women; has that inspired you to expand the type of models you use in your runway shows? Do you hope to be more inclusive moving forward?

Listen, from a media presence, I think our success has been that I've always dressed -- and I think I'm one of the few brands and designers that dresses -- women of all ages, women of all backgrounds, of all shapes and colors. I like to dress people that I think are really talented or have something interesting or iconic to them that inspires me in some capacity. That's how I've addressed it. And I don't follow rules -- I think that's the other thing in the documentary. It's not about following rules, everyone has their own journey.

I really love this season of Project Runway. I had a blast watching the girls on the runway, and the runway looks great. It was our best runway, I think. I'll continue to use all different types of models in the different work that I do. Actually, at the moment, I'm not doing runway shows. In the future, I may, but at the moment, I'm creating editorial stories and videos that I'm editing myself and there you have the ability, for sure, to play with your casting and do different casting. You know, my line with David's Bridal goes up to size 32. Somebody can come to my studio and order any size. Beauty is not in a size for me.